The need to extrapolate the biological effects of radiation to doses that are below the available experimental data is becoming increasingly important in radiation risk estimation and in radiation therapy. Current models used for these extrapolations are often inadequate dues to a lack of understanding of the basic mechanism of the biological actions of ionizing radiation. However, much theoretical and experimental data support the central importance of the microscopic features of radiation track structure in relation to biological action. In this regard, ultrasoft X-rays have significantly contributed to our understanding of the mechanistic aspects of ionizing radiation effects. Important questions remain regarding these data and their interpretation. The goal of this project is to employ the unique flexibility of the University of Wisconsin synchrotron as a source of nearly monochromatic ultrasoft X-rays of any desired energy to help solve these questions. Using our recently developed radiation biology synchrotron beamline, we will irradiate mammalian cells of various types (including V79, 10T1/2 and At) whose dimensions will be accurately measured by confocal microscopy with (a) isoattenuation groups of energies, i.e. X- rays with energy slightly above and below the carbon k-edge energy and measure cellular inactivation. Then, these data will be used to evaluate various dosimetric models with specific possible cellular locations of the critical targets for radiation cell killing. Similar experiments will also be conducted with repair deficient cell strains. Finally, we will further explore "site-size" effects by extending radiation survival measurements to lower X-ray energies that are currently available. These studies will thus provide data that will elucidate the physical size, critical energy and location of molecular targets for radiation cellular damage.